Is there no way to break down that huge atlas of yours? Does changing the device you are testing on changes the load times (like tetsing on a tablet instead of a phone, for instance)? I just checked and the load/unload atlas blocks only determine what will be loaded on the next scene. There might be a code way to load them mid-game, but then your game would freeze then instead of during the transitions, I think. I'm not well versed on atlases and whatnot, so I don't have much else to suggest.